1. Field of the Invention
This invention generally relates to integrated circuit (IC) fabrication and, more particularly, to a procedure for making a rare earth element-doped silicon oxide (SiO2) precursor with nanocrystalline (nc) Si particles, for use in silicon-based electroluminescence (EL) devices.
2. Description of the Related Art
The observation of visible luminescence at room temperature, emanating from porous silicon (Si), has spurred a tremendous amount of research into using nano-sized Si to develop a Si-based light source. One widely used method of fabricating nanocluster Si (nc-Si) is to precipitate the nc-Si out of SiOx (x<2), producing a film using chemical vapor deposition (CVD), radio frequency (RF)-sputtering, and Si implantation, which is often called silicon-rich silicon oxide (SRSO). Er implantation, creating Er-doped nanocrystal Si, is also used in Si based light sources. However, state-of-the-art implantation processes have not been able to distribute the dopant uniformly, which lowers the light emitting efficiency and increases costs. At the same time, there has been no interface engineering sufficient to support the use of such a dopant. The device efficiency is very low and the process temperature is very high, which limits the device applications. In order to improve the device efficiency, a large interface area must be created between nanocrystal Si and SiO2.
Silicon has conventionally been considered unsuitable for optoelectronic applications, due to the indirect nature of its energy band gap. Bulk silicon is indeed a highly inefficient light emitter. Among the different approaches developed to overcome this problem, quantum confinement in Si nanostructures and rare earth doping of crystalline silicon have received a great deal of attention. In particular, Si nanoclusters (nanocrystalline Si) embedded in SiO2 have in recent years attracted the interest of the scientific community as a promising new material for the fabrication of a visible Si-based light source. Alternatively, Er-doped crystalline Si has been extensively studied to take advantage of the radiative intra-4f shell Er transition. Room temperature operating devices with efficiencies of around 0.05% have been achieved. However as mentioned above, the device efficiency is very low and the process temperature is very high, normally over 1100° C.
Based on one theory for the photoemission of Si—SiO2 interface, Si 2p core-level shifts at the Si(001)-SiO2 interface depend linearly on nearest-neighbor oxygen atoms. Second nearest-neighbor effects turn out to be negligibly small. Therefore, the photoemission spectra require that all Si in the oxidation state be present at the interface. That is, the making of a large area of Si—SiO2 interface is a critical issue for EL device applications.
Other work (Castagna et al., “High Efficiency Light Emission Devices in Silicon”) describes a silicon-based light source consisting of a MOS structure with nc-Si particles and Er implanted in a thin oxide layer. The device shows 10% external quantum efficiency at room temperature, which is comparable to that of light emitting diodes using III-V semiconductors. The device consists of a 750 Å thick silicon-rich oxide (SRO) gate dielectric layer doped with rare earth ions (Er, Tb, Yb, Pr, Ce) via implantation. After annealing at 800° C. for 30 minutes under nitrogen flux, the implantation defects are eliminated and the agglomeration of silicon in the SRO film is obtained. The agglomeration of silicon, as matter of fact, forms the silicon nanoclusters, which act as a classic sensitizer atom to absorb incident photonics for the transfer of energy to luminescent Er3+ ions. The key feature of the silicon electroluminescent device is the SRO layer consisting of the nc-Si and the rare earth element doping. The nc-Si size is in the range of 10 to 30 Å.